As for SF lime juice ... I presume you mean limeade? Lime juice is so good for slowing down sugar into your blood stream, I don't think you need to worry about trace amounts of natural sugars in it.
Mojitos ... okay, let see if I can remember this just right:
<in a highball>
1 shot of white rum
1 sprig of mint, chopped
<mull with a spoon>
2 tablespoons Splenda
Juice of 4 limes (or 2 lemons)
<stir>
fill with ice
fill with club soda
<stir>
garnish with a sprig of mint